Site Editor should be available for child themes if the appropriate index.html file exists in the parent

[richtabor]

What problem does this address?

Currently a theme requires a block-templates/index.html file to enable the Site Editor UI (which makes sense). However, if a child theme of the block based theme does not have an index.html file, it is opted-out of the experience. This means that child themes will have to duplicate index.html files just to be opted into the Site Editor (which isn't ideal if a child theme does not change the index blogroll view).

What should occur?

A parent theme's block-templates/index.html file should be available within child themes, which means child themes of Site Editor capable themes can use the Site Editor without having to duplicate that index file.

[Mamaduka]

Related #25612.